How do I unlock an iCloud locked MacBook from a previous owner?

You don't.

The whole purpose of the cloud lock (activation lock) is to prevent others from accessing the computer and files if it is lost or stolen. The only one who can release it from the activation lock is the previous owner. There is no workaround. This is why they let you have it free.

The previous owner should have done this: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

The lesson here is that used Macs will not be accessible unless properly prepared by the previous owner. Check before you buy. And never buy from questionable sources: eBay, Craigslist, LetGo, etc.

Activation lock is a iCloud security feature to prevent people other than the original owner from accessing an Apple device. Either the original owner didn’t erase and remove their Apple ID properly or it’s a stolen/lost device.

In Apples eyes you are not the rightful owner unless the original owner removes their account for you. The store you bought it from should have never sold it to you. Return it to where you bought it for a refund.